Characterization For The Structures And Properties Of Wide Bandgap Semiconductor In₂O₃ By First-principle Calculations

Wide-gap semiconductor materials have exhibited unique physical and chemical properties in the past few decades, giving rise to numerous potential applications in electronics, chemistry, physics and biology. Thus the investigation on these materials has become an active field. Experimentally, the preparation of new and excellent materials has attracted many researchers' interesting; on theory, using quantum chemical theory and computer technology, the calculations for the electronic structures and optical properties of semiconductor materials have been the researching orientation. At this wok, we investigate the electronic structures and optical properties of wide-gap semiconductor In₂O₃ by using a first-principle calculation based on the density functional theory (DFT).Main contents of this dissertation include:1. The calculations and analysis of the structures and optical properties of pure In₂O₃.Using CASTEP code in the Material Studio software, we have studied the geometrical structure,electronic structure and optical properties for cubic and rhombic In₂O₃. In our computed results, the bonding between In and O mainly consists of electrovalent bond, but there is still little covalent bond in the crystal. From the density of states and band structure, we can ananlyze and identify the optical absorption and transition described by the imaginary part of the dielectric function. We can see that the peak at 6eV mainly originate from the electron transition between valence band O 2p and conduct band In 5s; the peak at about 9eV responds to the transition from O 2p to In 5s or 5p states. And additionally, in visible light area 0～5eV the absorption is very weak, so we think In₂O₃ crystal is a good transparent material.2. The investigation of the changes for the electronic structures and photocatalytic properties of N-doped and N:H-coped In₂O₃ by first-principle calculations.(a) Using DMol3 to analyze the action of N impurities on energy band structure for In₂O₃. For our results, different N impurities types have different mechanism on electronic structures. When N substitutes O in the lattice, N 2p states lie above the valence band maximum and have hybridization with O 2p states. The electron transition energy from N 2p impurity states to the conduct band minimum is lower than pure phase, and that is the reason of the absorption edge red-shift for N doped In₂O₃. For interstitial N-doping, N and O form NO^- species, and there are two NO anti-Ï€orbital in the band gap. In addition, because of the different electronic density between cubic and rhombic In₂O₃, the influence of N impurities is different between two phases—the changes as a result of N-doping in rhombic models are more significant than that in cubic models.(b) Through first-principle calculations, we have studied the changes of electronic structures in N:H-codoped models. Our computed results show that H atom is donor impurity to supply electron when N:H-codoping. In substituted doped models, the energy of N 2p states is reduced and more hybridization between N 2p and O 2p; when NH interstitial doping, the number of impurity energy levels in the gap is decreased, there is only one NO anti-Ï€orbital and the band gap is reduced.3. Researches for the intrinsic defects in cubic In₂O₃ through DFT calculations.In the present paper using DFT calculations we have analyzed the structures, formation energy and electronic structures for the intrinsic defects of cubic In₂O₃. The calculations of formation energy show the oxygen vacancies are the most easily formed both poor and rich conditions. The other most stable defect is dependent on the oxygen partial pressure. Oxygen vacancies result in a donor energy level below conduct band bottom, and meanwhile Fermi energy shift to CBM. The oxygen vacancies are the main factor on n-type conductivity in In₂O₃; for oxygen interstitials, when the structure comes to stable state, O₂^2- has been formed. In this structure, band-gap is significant decreasing and two anti-Ï€orbitals lie above the valence band; in vacancies are acceptor defects, and the defect energies are near to VBM. In vacancy is a p-type defect. But due to the higher formation energy, its contribution to conductivity of In₂O₃ is no obvious; there are two donor levels in indium interstitials. The shallow level is occupied by one electron and pass through Fermi level, while the deep level is occupied by two electrons and below Fermi level. For our calculation, the oxygen interstitials are favorable for optical absorption of In₂O₃. From the literatures, we find that In₂O₃ are more investigated in an experimental way and most attention on its transparent conductivity, theoretical study on its photocatalytic property is scare. And through geometrical structure, electronic structure, Mulliken analysis and optical properties, especially the bonding and states distribution of impurity, the paper explains the mechanism of N-doping and N:H-coping in the In₂O₃ crystal. In this study, we have, on the one hand, treated the problems with semiconductor physics means, and on the other hand, tried to link chemical theory, in order to make a new sight and deeper understanding for our jobs.